This invention relates to an electromagnetically operable electric switch of the kind having at least one breaking unit comprising two electrically series-connected contacts.
In low voltage switching equipment it is customary to employ a plurality of series-connected devices in one and the same current supply circuit. For example, the supply circuit for an electric motor may include a load switch, fuses, a contactor, an overload relay and a safety switch. Such a circuit has the disadvantage that it is difficult to coordinate the various switching devices with the short-circuit protection members in a functional manner. Other disadvantages are that the devices occupy a relatively large space when mounted in a switchgear cubicle, that they have a large number of connection points and that they have relatively high power losses.
1. Field of the Invention
To overcome the above-mentioned disadvantages, attempts have been made to combine different functions in one and the same device, for example by constructing a contactor so that it also operates as a current-limiting circuit-breaker (see e.g. German Auslegeschrift No. 1 194 956 and Swedish Published Patent Application No. 7714933-4). However, with the contact means that have been proposed for such devices, it is difficult to achieve simultaneously a high breaking capacity and a long service life.
2. Description of the Prior Art
The present invention aims to provide an electric switch which combines the functions of a contactor and a circuit-breaker and which has a long electrical and mechanical life and is capable of dealing with short-circuit interruptions.
According to the invention, in an electromagnetically operable electric switch having at least one breaking unit comprising first and second electrically series-connected openable contact systems, the first contact system comprises a movable first contact, and electromagnetic means controlling engagement of the first contact with a fixed second contact, and the second contact system comprises a movable third contact normally engaged with the fixed second contact and disengageable from the fixed second contact only in response to a current through the switch considerably in excess of the rated current of the switch, the fixed second contact being disposed intermediate the first and third contacts.
In a preferred embodiment of the switch in accordance with the invention, the contacts of the first contact system are made of a first material having properties suitable for making and breaking currents occurring during normal operation of the switch, and the contacts of the second contact system are made of a second material having properties suitable for interrupting short-circuit currents.